Posts By: Lin Li, MSc, PhD

We have discussed the association between ADHD and obesity in our first blog (https://newbrainnutrition.com/adhd-and-obesity-does-one-cause-the-other/), briefly summarized, evidence from various study designs suggested that shared etiological factors might contribute to the above association. Recently, a large genome-wide association study (GWAS) on risk genes for ADHD reported a significant genetic correlation between ADHD and a higher risk of overweight and obesity, increased BMI, and higher waist-to-hip ratio, which further supported that there could be genetic overlap between obesity and ADHD (1).

Considering the previously described occurrence of unhealthy dietary intake in children and adolescents with ADHD in our second blog (https://newbrainnutrition.com/unhealthy-diets-and-food-addictions-in-adhd/), along with the fact that bad eating behaviours are crucial factors for the development of obesity, We can speculate that the shared genetic effects between ADHD and unhealthy dietary intake may also explain the potential bidirectional diet-ADHD associations. Is there any available evidence to support the above hypothesis?

To date, dopaminergic dysfunctions underpinning reward deficiency processing (or neural reward anticipation), was reported as a potential shared biological mechanism, through which the genetic variants could increase both the risk for ADHD and unhealthy dietary intake or obesity. Via the Gut-Brain axis, a two-way and high-speed connection, the gut can talk to the brain directly. According to the study (2), a higher proportion of bacteria that produce a substance that can be converted into dopamine was found in the intestines of people with ADHD than those without ADHD. Using functional magnetic resonance imaging (fMRI), they further found that the participants with more of these bacteria in their intestines displayed less activity in the reward sections of the brain, which constitutes one of the hallmarks of ADHD. We are therefore proposing the idea that there could be a biological pathway- ‘dietary habits-gut (microorganism)-reward system (dopamine)-ADHD’, through which the shared genetic effects between ADHD and unhealthy dietary intake may play a role.

In order to determine whether the genetic overlap between ADHD and dietary habits actually exists, we will in our next Eat2beNice project use twin methodology and unique data from the Swedish Twin Register. We will keep you updated!

This was co-authored by Henrik Larsson, professor in the School of Medical Science, Örebro University and Department of Medical Epidemiology and Biostatistics, Karolinska Institute, Sweden.

Recent research (1,2) on children and adolescents has reported that elevated levels of ADHD symptoms are positively associated with unhealthy dietary habits, including a higher consumption of refined sugars, processed food, soft drink, instant noodles, and a lower intake of vegetables and fruits. However, the link between low-quality diets and risk of ADHD in adults is still not well established, which would be further explored in the ongoing Eat2beNICE research project.

What is the underlying mechanism for an association between ADHD and unhealthy dietary habits? There is still no clear answer. Nemours’ potential biological pathways, by which dietary intake could have an impact on mental health, has been proposed in the literature (2). For example, iron and zinc are cofactors for dopamine and norepinephrine production (essential factors in the etiology of ADHD), so unbalanced diet with lower levels of iron and zinc may further contribute to the development of ADHD. However, we cannot overlook the possibility of a bi-directional relationship between diet quality and ADHD, especially when the interest in the concept of “food addiction” has received increased attention.

Food addiction refers to being addicted to certain foods (e.g. highly processed foods, highly palatable foods, sweet and junk foods) in a similar way as drug addicts are addicted to drugs. Animal models (3) have suggested that highly processed foods may possess addictive properties. Rats given high-sugar or high-fat foods display symptoms of binge eating, such as consuming increased quantities of food in short time periods, and seeking out highly processed foods despite negative consequences (e.g. electric foot shocks). One human study (4) found that individuals with high levels of ADHD-like traits (e.g. high levels of impulsively, disorganised, attention problems) were more likely to suffer from problematic eating behaviour with overconsumption of specific highly palatable foods in an addiction-like manner. Therefore, food addiction may, just as substance abuse, be over-represented among individuals with ADHD.

Thus, it seems there could be a vicious cycle between unhealthy dietary habits and ADHD: ADHD may lead to a worse choice of diet, lowering the health quality, which could eventually exacerbate ADHD symptoms. We will further test the bidirectional diet-ADHD associations in the ongoing Eat2beNice project.

This was co-authored by Henrik Larsson, professor in the School of Medical Science, Örebro University and Department of Medical Epidemiology and Biostatistics, Karolinska Institute, Sweden.

AUTHORS:
Lin Li, MSc, PhD student in the School of Medical Science, Örebro University, Sweden.
Henrik Larsson, PhD, professor in the School of Medical Science, Örebro University and Department of Medical Epidemiology and Biostatistics, Karolinska Institute, Sweden.

Attention-deficit/hyperactivity disorder (ADHD) is a common neurodevelopment disorder characterized by inattention or hyperactivity–impulsivity, or both. It might seem paradoxical, but many studies indicate that individuals with a diagnosis of ADHD suffer from overweight and obesity. Therefore, it is important to understand the underlying mechanism that put individuals with ADHD at risk for obesity.

Evidence from within-individual studyA systematic review and meta-analysis (1) based on 728,136 individuals from 42 studies, suggested a significant association between ADHD and obesity both in children/adolescents and adults. The pooled prevalence of obesity was increased by about 70% in adults with ADHD and 40% in children with ADHD compared with individuals without ADHD. However, due to the lack of longitudinal and genetically-informative studies, the meta-analysis was unable to explain the exact direction of association and the underlying etiologic mechanisms. There are several potential explanations:

ADHD causing obesity: The impulsivity and inattention components of ADHD might lead to disordered eating patterns and poor planning lifestyles, and further caused weight gain.

Obesity causing ADHD: Factors associated with obesity, for example dietary intake, might lead to ADHD-like symptoms through the microbiota-gut-brain axis.

ADHD and obesity may share etiological factors: ADHD and obesity may share dopaminergic dysfunctions underpinning reward deficiency processing. So the same biological mechanism may lead to both ADHD and obesity. This is difficult to investigate within individuals, but family studies can help to test this hypothesis.

We will further investigate these possibilities in the Eat2beNICE research project by using both perspective cohort study and twin studies.

Evidence from a recent within-family studyRecently, a population-based familial co-aggregation study in Sweden (2) was conducted to explore the role of shared familial risk factors (e.g. genetic variants, family disease history) in the association between ADHD and obesity. They identified 523,237 full siblings born during 1973–2002 for the 472,735 index males in Sweden, and followed them until December 3, 2009. The results suggest that having a sibling with overweight/obesity is a risk factor for ADHD. This makes it likely that biological factors (that are shared between family members) increase the risk for both ADHD and obesity.

Evidence from across-generation studyGiven that both ADHD and obesity are highly heritable complex conditions, across-generation studies may also advance the understanding of the link between ADHD and obesity.

A population-based cohort study (3) based on a Swedish nationwide sample of 673,632individuals born during 1992-2004, was performed to explore the association between maternal pre-pregnancy obesity and risk of ADHD in offspring. The sibling-comparison study design was used to test the role of shared familial factors for the potential association. The results suggest that the association between maternal pre-pregnancy obesity and risk of ADHD in offspring might be largely explained by shared familial factors, for example, genetic factors transmitted from mother to child that contribute to both maternal pre-pregnancy obesity and ADHD.

Together, based on previous evidence from various study designs, there is evidence to suggest that the association between ADHD and obesity mainly is caused by shared etiological factors. However, future studies on different population are still needed to further test these findings.

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